Coronal magnetic loops may display a wide variety of flow
configurations, depending upon their geometries and the
plasma conditions at their footpoints, as well as upon the
distribution of nonthermal heating along them. Existing
models of steady state loop flows ("siphon" flows, such as
may exist in the closed-field regions of large, long-lived
helmet streamers) show that the bulk plasma streaming velo-
city at the top of a loop can easily reach values of order
the acoustic speed, i.e., in excess of 100 km/s, and may be
even higher in the supersonic (descending) leg of the loop.

In this paper we consider some of the observational con-
sequences of such siphon flows. Using a simple polytropic
model to describe the variation of plasma parameters along
a typical coronal loop, we have calculated the expected
frequency-dependent emissivities of the resonance-scattered
H I Lyman-alpha and O VI emission lines as functions of
position on the loop and of the loop orientation relative to
the viewing direction. Comparison with similar calculations
for a static plasma with the same temperature and density
provides the degree of Doppler dimming to be expected at
various points along the loop. This Doppler dimming, and
the related Doppler shift, comprise important diagnostic
signatures of loop flows which are potentially observable
with the UVCS instrument on SOHO. Surprisingly, Doppler
dimming, which is ordinarily regarded as an indicator of
radial outflow from the Sun, does not vanish entirely for
the horizontal flow at the top of a loop. For a hot (T
2 MK), nearly isothermal loop extending less than, say, 1
solar radius above the limb, the Doppler dimming there can
achieve values of more than 10 per cent at the loop's
summit.